Acquiring signals indicative of the health of a fetus, both during gestation and during labor, assists in the treatment of a pregnant subject to insure healthy growth and delivery of the fetus. A determination of the heart health profile of a fetus may be used to screen for developmental heart defects, and, as fetal hypoxemia can alter the shape of the fetal electrocardiogram (fECG) waveform, the ability to acquire the electrocardiogram of a fetus is particularly desirable during labor to determine whether the fetus is getting enough oxygen. Such information assists an obstetrician in making informed health care decisions, such as whether to expedite delivery by means such as Caesarian section, in order to prevent or reduce the risk of severe metabolic acidosis and neonatal encephalopathy, which can result in cerebral palsy or other undesirable outcomes. Reliable acquisition of fetal ECG signals is critical to the identification of characteristic ECG patterns that detect existent fetal injury or predict impending or fetal injury caused by inflammatory, hypoxic, or ischemic insults. Presently-used invasive methods for fetal ECG monitoring are only conducted during labor, so there exists no ECG method of screening for fetal heart defects. Earlier, more accurate determination of such defects might lead to quicker repair of such defects after birth, or even before birth.
Non-invasive acoustic devices such as stethoscopes and microphones have long been used to determine fetal heart rate by listening for and counting a fetal heartbeat. Such methods do not present all the information found in a fetal electrocardiogram. Devices such as the cardiotocograph and the Doppler fetal monitor use an ultrasound transducer to detect fetal heartbeat and determine fetal heart rate, but such devices are labor intensive and inherently unreliable because they require regular adjustment of an ultrasound transducer over a fetal artery or heart valve, such placement being prone to change due to fetal or maternal movement. In any case, these devices do not acquire an electrocardiogram.
The fetal scalp electrode method involves passing an electrode through the pregnant subject's cervix and attaching it to the head of the fetus. Use of such a method is limited to the final stages of labor when the fetal scalp is accessible through the birth canal following the rupture of the amniotic membranes and the dilation of the cervix, and moreover presents the disadvantages typical of invasive diagnostics, e.g., complexity and expense of setup, subject discomfort, potential for harm to the fetus, and risk of infection. Cases of fetal scalp abscess have been reported after use of this method.
Although an accurate model of fetal heart activity can be constructed, the acquisition of a true representation of a fetal ECG from electrophysiological signals acquired by electrodes non-invasively placed on the skin of the maternal torso is impeded by absorptive media that exist between the fetus and the subject's abdominal surface and by the presence of the stronger maternal ECG that must be filtered out to remove interference. Techniques for isolating the fetal cardiac signal have remained as simple as template subtraction, matched filters, averaging and principal component analysis (PCA).
Experimental non-invasive fetal electrophysiology monitoring systems are cumbersome because of the setup time involved in the placement of the electrodes. NeuroDimensions Incorporated has been developing fECG algorithms, and has used 8-12 gel-based ECG electrodes for their acquisition apparatus. This number of ECG electrodes is inadequate to permit acquisition of ECG signals in a sufficient number of electrical axes so as to extract and distinguish fetal ECG and maternal ECG from the acquired signal. The portable fetal heart rate monitoring system CARE 2000, developed by Monica Healthcare, records the very small electrical signals generated by the fetal heartbeat and other electrophysiological events present on the maternal abdomen. The Care 2000 has demonstrated excellent correlation of the extracted fetal heart rate in comparison with conventional Doppler ultrasound fetal heart recording. The electrophysiological signals can also be used to extract fetal position, uterine activity information, and fetal ECG morphology parameters. However, this device cannot identify or assess the entire ECG waveform.
U.S. Pat. No. 3,703,168 to Frink describes a plurality of surface electrodes attached to a mother's skin and signal processing apparatus for distinguishing fetal rhythm ECG. U.S. Pat. No. 4,781,200 to Baker describes mounting a plurality of acoustic sensors and a movement sensor to a belt for detecting fetal heart rate, and while the disclosure contemplates using electrocardiographic electrodes in lieu of acoustic sensors, it does not envision acquiring a fetal electrocardiogram, only using sensors to obtain fetal heart rate. Baker also does not envision extraction and distinction of various types of electrophysiological signals from each other, such as ECG from a signal dominated by EMG, or vice versa.
Gel-type electrodes include a hydrating gel for reducing impedance of electrode-skin contact. Because this impedance increases as the hydrating gel dries, ultimately to the point where artifact dominates useful electrophysiological signal, these electrodes have a limited shelf life and a relatively short useful lifetime once applied. Application of gel-type electrodes also typically involves preparation of the electrode placement site by emery abrasion of the stratum corneum layer of the skin. For this reason and because of the adhesives typically used to secure them to the skin site, gel-type electrodes are generally not repositionable after they have been placed. They can become uncomfortable after extended wear due to the skin abrasion and due to irritation caused by the tape.
It is therefore the object of the present invention to offer a comfortable, convenient, wearable monitoring apparatus that makes both short-term and long-term non-invasive fetal and maternal biosignal monitoring simple and inexpensive. Specifically, it would be desirable to have a monitoring apparatus that could be easily put on and taken off without special training that would nevertheless be capable of reliably acquiring high-quality electrophysiological signals from both the fetus and the mother. Additionally, it is the object of the present invention to offer an electrode harness in the form of a wearable with sensor design and placement adapted to the acquisition of fetal and maternal ECG signals. Additionally, it is the object of the present invention to provide a method of fetal ECG acquisition useable at any stage of gestation or delivery. It is further the object of the present invention to offer an integrated, resizable electrode harness that uses dry electrodes with an unlimited shelf life and a much longer useful lifetime once applied, and which also reduces setup time by eliminating the need for skin preparation. Finally, it is the object of the present invention to offer an electrode harness that allows advanced assessment of the fetal ECG waveform characteristics, which will mitigate risk to the fetus during delivery.